Ethylbenzene is a key raw material in the production of styrene and is produced by the reaction of ethylene and benzene in the presence of an acid alkylation catalyst. Older ethylbenzene production plants, those preferably built before 1980, used AlCl3 or BF3 as the acidic alkylation catalyst. Plants built after 1980 have in general used zeolite-based acidic catalysts as the alkylation catalyst.
Three types of ethylation reactor systems are used for producing ethylbenzene, namely, vapor phase reactor systems, liquid phase reactor systems, and mixed phase reactor systems.
In vapor-phase reactor systems, the ethylation reaction of benzene and ethylene is carried out at a temperature of about 350 to 450° C. and a pressure of 690-3534 KPa-a (6-35 kg/cm2-g) in multiple fixed beds of zeolite catalyst. Ethylene exothermically reacts with benzene in the presence of such zeolite catalyst to form ethylbenzene, although undesirable reactions also occur. About 15 mol. % of the ethylbenzene formed further reacts with ethylene to form di-ethylbenzene isomers (DEB), tri-ethylbenzene isomers (TEB) and heavier aromatic products (heavies). All these undesirable reaction products, namely DEBs, TEBs and heavies, are often commonly referred to as polyethylated benzenes (PEBs).
By way of example, vapor phase ethylation of benzene over a catalyst comprising crystalline aluminosilicate zeolite ZSM-5 is disclosed in U.S. Pat. Nos. 3,751,504 (Keown et al.), 3,751,506 (Burress), and 3,755,483 (Burress).
In recent years the trend in industry has been to shift away from vapor phase ethylbenzene reactors in favor of liquid phase reactors. Liquid phase reactors operate at a temperature of about 150-280° C., which is below the critical temperature of benzene (290° C.). The rate of the ethylation reaction in a liquid phase reaction is lower as compared to a comparable vapor phase reaction. Hence, the catalyst volumes required for the liquid phase reaction are greater than those for the vapor phase reaction, but the lower design temperature of the liquid phase reaction economically compensates for the negatives associated with the higher catalyst volume.
Liquid phase ethylation of benzene using a catalyst comprising zeolite beta is disclosed in U.S. Pat. No. 4,891,458 and European Patent Publication Nos. 0432814 and 0629549. More recently it has been disclosed that MCM-22 and its structural analogues have utility in these alkylation/transalkylation reactions, for example, U.S. Pat. No. 4,992,606 (MCM-22), U.S. Pat. No. 5,258,565 (MCM-36), U.S. Pat. No. 5,371,310 (MCM-49), U.S. Pat. No. 5,453,554 (MCM-56), U.S. Pat. No. 5,149,894 (SSZ-25); U.S. Pat. No. 6,077,498 (ITQ-1); International Patent Publication Nos. WO97/17290 and WO01/21562 (ITQ-2).
Typically, the zeolite catalysts employed in hydrocarbon conversion processes, such as aromatics alkylation, are in the form of cylindrical extrudates. However, it is known from, for example, U.S. Pat. No. 3,966,644 that shaped catalyst particles having a high surface to volume ratio, such as those having a polylobal cross-section, can produce improved results in processes which are diffusion limited, such as the hydrogenation of reside.
Moreover, it is known from U.S. Pat. No. 4,441,990 that a polylobal catalyst particle having a non-cylindrical centrally located aperture can reduce the diffusion path for reagents and the pressure drop across packed catalyst beds while minimizing catalyst loss due to breakage, abrasion and crushing. In particular, Example 8 of the '990 patent discloses that hollow trilobal and quadrilobal ZSM-5 catalysts are more active and selective for the ethylation of benzene at 770° F. and 300 psig pressure than solid cylindrical catalysts of the same length. Under these conditions, the reagents are necessarily in the vapor phase.
U.S. Pat. No. 6,888,037 disclosed a process for producing cumene which comprises the step of contacting benzene and propylene under at least partial liquid phase alkylating conditions with a particulate molecular sieve alkylation catalyst, wherein the particles of said alkylation catalyst have a surface to volume ratio of about 80 to less than 200 inch−1. According to U.S. Pat. No. 6,888,037, the liquid phase propylation of benzene, unlike the liquid phase ethylation of benzene, is sensitive to intraparticle (macroporous) diffusion limitations. In particular, by selecting the shape and size of the particles of the alkylation catalyst such that the surface to volume ratio is within the specified range, the intraparticle diffusion distance can be decreased without excessively increasing the pressure drop across the first catalyst bed. As a result, the activity of the catalyst for the propylation of benzene can be increased, while at the same time the selectivity of the catalyst towards undesirable polyalkylated species, such as diisopropylbenzene (DIPB) can be reduced.
Surprisingly, however, it has now been found that size and shape of the catalyst particles beyond the range disclosed in the U.S. Pat. No. 6,888,037 can yield improved results, e.g., activity and/or selectivity in the liquid phase alkylation of benzene to produce monoalkylbenzene, such as ethylbenzene and cumene.